Science - USA (2020-03-13)

lower temperatures (1.8 K in Fig. 1C), two ad-
ditional small plateaus with a jump of ~1/6Ms
appear at 0.9 and 3.2 T, respectively, accom-
panied by a small hysteresis. TheM(H) curve
forH//aalso shows well-defined plateaus,
albeit at different ranges of field (fig. S2B),
whereas no plateaus are observed forH//c(fig.
S2E). AnH–Tphase diagram based on temper-
ature dependence of the peaks in field derivative
ofM(H)curves(fig.S2,CandD)isconstructed
in Fig. 1D. Together with thec(T)dataabove,
the lack of any clear magnetic transitions for
H//cconfirms that the Ho spins in HoAgGe
are constrained in theabplane and have ad-
ditional in-plane anisotropies, similar to those
in the isostructural compounds TmAgGe and
TbPtIn ( 30 ).

Magnetic structures determined from
neutron diffraction

To fully determine the nontrivial spin struc-
tures of HoAgGe, we performed single-crystal
neutron diffraction experiments down to 1.8 K
and underH//bup to 4 T ( 31 ). Below the high-
temperature transitionT 2 =11.6K,amagnetic
peak appears at (1/3, 1/3, 0) (Fig. 2A and fig.
S4A), indicating a√3×√3magneticunitcell
(the green rhombus in Fig. 2B). Below 10 K,
mostnuclearsitesexhibitalmostconstantin-
tensity but the broad transition atT 1 induces
additional magnetic contribution at certain
structural diffraction sites, such as (1, 0, 0)
(Fig. 2A, inset).
According to neutron data at 10 K (fig. S5A),
the magnetic structure belongs to the mag-
netic space group P-6′m2′(Table 1), which has
three nonequivalent Ho sites labeled by Ho1,
Ho2, and Ho3 in Fig. 2B. Six other Ho positions
in the magnetic unit cell are obtained from
above three Ho sites by threefold rotations
around thecaxis. Because there are no mag-
netic contributions at nuclear sites at 10 K, the
simplest possibility for (MHo1,MHo2,MHo3)is
(M,–M, 0), withMdetermined to be 5.2(1)mB
(Table 1 and fig. S10B). This corresponds to
Ho1 and Ho2 exhibiting ordered moments of
thesamesizebutoppositedirectionsand
Ho3’s moment fluctuating without order-
ing. Such a partially ordered magnetic struc-
ture is shown in Fig. 2C, with the ordered
moments forming clockwise or counterclock-
wise hexagons separated by the unordered
moments. The structure thus has a nonzero
magnetic toroidal moment defined byt¼
1
V∫d

(^3) rrM( 32 ). Similar partially ordered
structures have also been observed in the
isostructural Kondo lattice CePdAl below
2.7 K with easyc-axis anisotropy ( 33 )andin
hexagonal UNi 4 B below 20 K, with 2/3 of
U moments forming in-plane clockwise hex-
agons ( 34 ).
BelowT 1 ~7 K, Ho3 moments also enter the
long-range order, as indicated in the inset of
Fig. 2A. Refinement of neutron data at 4 K (figs.
S5B and S10C) leads to the magnetic structure
shown in Fig. 2E, which also has the P-6′m2′
symmetry, with (MHo1,MHo2,MHo3)=(M,–M,

• M)andM=7.5(1)mB. As illustrated in Fig. 2F,
  this fully ordered ground state includes alter-
  nating clockwise and counterclockwise hexa-
  gons of spins, and another 1/3 of hexagons
  consisting of three pairs of parallel spins.
  This is exactly the√3×√3 ground state of the
  classical kagome spin ice predicted theoret-
  ically before ( 35 – 37 ).

To confirm that HoAgGe is indeed a kagome

spin ice, however, it is necessary to show that

the ice rule is established even outside the

fully ordered ground state ( 9 – 11 ). The kagome

ice rule requires dominating nearest-neighbor

ferromagnetic coupling between coplanar spins

with site-dependent Ising-like uniaxial anisotropy

( 9 – 11 ). Using neutron diffraction under mag-

netic fields, we show that these requisites are

indeed satisfied in HoAgGe. Figure 2D displays

the neutron-scattering integrated intensities of

Zhaoet al.,Science 367 , 1218–1223 (2020) 13 March 2020 2of6

b

a

10 20 30 40

0.2

0.4

0.6

0.8

1.0

T 2

T(K)

HoAgGe; H=500Oe

H//b

H//c

T 2

4 8 12 16 20 24

0.00

0.05

0.10

T 1

χ

)lom/ume( dχ/dT(emu/mol K)

T(K)

A B

C D

HoAgGe; H//b

1.8K(H up)

1.8K(H down)

5K

8K

10K

12K

15K

0246

0

2

4

6

μ 0 H(T)

M(μ

/Ho)B

HoAgGe; H//b

2 4 6 8 10 12

0

1

2

3

4

μ^0

H(T)

5/6 plateau

Ground state

Saturated state

1/6 plateau

2/3 plateau

T(K)

1/3 plateau

Fig. 1. Crystal structure and magnetic properties of HoAgGe.(A)c-Axis projection of the HoAgGe

crystal structure, with the definition ofaandbdirections. (B) Low-temperature susceptibilityc(T)of

HoAgGe for bothH//bandH//cunder 500 Oe, withdc(T)/dTin the inset. (C) Isothermal in-plane (H//b)

magnetization for HoAgGe at various temperatures. (D) Dependence of the metamagnetic transitions on

temperature, with the dotted line indicatingT 1 (see text).

Table 2. The four low-energy (<1 meV) CEF modes of Ho3+in HoAgGe.

Irreducible representation Wave functions

G 2 −0.6186(|7> +|−7>)−0.1871(|5> +|−5>)−0.2591(|3> +|−3>)

.....................................................................................................................................................................................................................+ 0.1234(|1> +|−1>)

G 4 −0.6209(|7>−|−7>)−0.1961(|5>−|−5>)−0.2636(|3>−|−3>)

.....................................................................................................................................................................................................................−0.0814(|1>−|−1>)

G 3 −0.0780(|8>−|−8>)−0.6256(|6>−|−6>)−0.1512(|4>−|−4>)

.....................................................................................................................................................................................................................−0.2822(|2>−|−2>)

G 1 0.0938(|8> +|−8>) + 0.6472(|6> +|−6>) + 0.1865(|4> +|−4>)

.....................................................................................................................................................................................................................+ 0.1257(|2> +|−2>)−0.2083|0>

RESEARCH | RESEARCH ARTICLE